
Dillocircus 
Assistant
cub appreciator
I think stuff like protogen (sergals, chakats, synths, aeromorphs…) is fairly okay (not clear if aeromorphs are okay for this site though) as they are relatively well-known within the fandom.
If someone has a unique species of their own that other people are not actually using though? We’d just tag that as whatever broader tag works, like canine/mammal/alien etc. plus original species. I’d include in the latter category most of the original species from Rick Griffin’s sci-fi work, for example. I’m personally undecided about Cobalt’s Foozles, I’d probably ask Pup for a final decision one way or another if we had more than, like, one pic of them XD.
In this case, I think the e621 approach of not tagging original species in particular way (though there’s several nonstandard tags floating about over there) is sensible, though pup may eventually decide otherwise.